1. Field of the Invention
The invention relates to apparatus such as pressure sensors which include a force transducer having a resonating silicon beam formed in a silicon wafer, and particularly to a method for forming the same.
2. Discussion of Prior Art
U.S. Pat. No. 5,090,254 to Guckel discloses a polysilicon resonating beam transducer. The transducer includes a beam of polysilicon which is mounted at its ends in a silicon substrate or wafer. The substrate and a polysilicon shell cooperate to define a cavity around the beam. The cavity is sealed from the atmosphere and evacuated to maximize the Q of the resonating beam. Resonant excitation of the beam can be accomplished in various ways, such as by capacitive excitation. Vibratory motion of the beam may be detected using an implanted resistor which is piezoresistive. Formation of the beam is carried out by depositing the beam on a sacrificial layer and surrounding the beam in a second sacrificial layer before the encapsulating polysilicon shell is formed. The sacrificial layers are etched out with liquid etchant which passes through channels in the periphery of the shell. Following etching, the interior of the cavity surrounding the beam is maintained in a wash liquid so that the beam is not deflected toward any of the adjacent surfaces, and the wash liquid is removed by freezing and sublimation. The interior surfaces of the cavity and the outer surfaces of the beam are passivated, and the channels leading into the cavity are sealed by oxidation in an oxidizing atmosphere, which also results in consumption of oxygen in the cavity.
U.S. Pat. No. 5,242,863 to Xiang-Zheng et al. relates to a silicon diaphragm piezoresistive pressure sensor having a diaphragm formed by a single-sided fabrication method. The diaphragm is formed with a cavity thereunder by: a) forming a buried low resistance layer under a predetermined diaphragm region; b) epitaxially depositing a layer of silicon; c) forming low resistance trenches through the epitaxial layer which trenches are connected with the buried low resistance layer at the edge of the buried low resistance layer; d) converting the low resistance trenches and buried low resistance layer to porous silicon by anodization in a concentrated HF solution; e) removing the porous silicon by etching; and f) filling the openings formed by etching with polysilicon to seal the cavity.